Hard Drive won't mount in Yosemite

In disk utility I only see 1tb wd drive. My partitions dont show up. Hence repair options etc aren't there. I tried resetting pram and Smc didn't help.

I had the same problem, had 2 Toshiba external 4TB hard drives (one of which had my backup on it) and a WD, all working fine until a couple of days ago when my iMac crashed. Had to do a total system restore to original settings (with new Yosemite update), and when I restarted none of my hard drives showed up in finder, even though I could see them in Disk Utility. Tried a bunch of things listed in this forum to no avail. In the end, I downloaded a trial copy of http://www.tuxera.com/products/tuxera-ntfs-for-mac/download/ and now everything is working fine.

So I have had this issue come up for a number of friends and acquaintances and I have been able to resolve the issue every time by connecting it to my Windows based machine (Windows 8.1) and repairing it through CHKDSK. I had no luck trying to repair it within Disk Utility on 10.10

I am not sure that this will work for every drive having a similar issue and there may well be circumstances that I am not aware of that led to my success. I hope this helps everyone

In Windows 8.1

1 Either the OS will automatically recommends fixing the drive in which case you just have to do the next option, just allow it to run the fix

If Not

2 Open search from the "Charms" menu and type "Command Prompt"

Type:

chkdsk Z: /f /x /r ------- Replace Z: with the drive letter that corresponds to your external drive

Wait, then wait some more (It can take a long long time)

With any luck it completes successfully and you can then use your external drive again.

I have a Toshiba 500G USB 3.0 external drive as a Time Machine backup for my iMac from 2009, which works under Snow Leopard.

When I got my new iMac5K, I am so happy to plug my Time Machine backup to migrate my system.

Unfortunately, the LED on the drive did light up as blue (running on USB3) but the iMac5K did not recognize any drive.

The iMac5K would not mount the disk and the Disk Utility did not show the disk.

The drive is perfectly functional, since my old iMac still works with it.

Updated everything in App Store on Yosemite, tried NVRAM reset and power cycle, not working.

The work-around is to plug that drive to a USB2.0 hub (the keyboard would also do) which connects to the new iMac 5K.

The drive LED lighted up as white (running on USB2) and the iMac 5K automatically mounted the disk.

So far, I cannot get any USB3.0 device work as USB3 on my iMac 5K.

Any suggestions?

What works when this happens to me:

The drive usually shows up in Disk Utility. Click on "Mount" Icon on tool bar at top. Might take a few seconds, but that's all it is for me.

I had the same problem but because of this discussion it helped me fix the same problem. WD 1 TB external hard drive won't mount on my mac mini Yosemite, I had very important video project files, photos, stock elements and I really really need to access them. So I tried to repair under disk utility and nothing happened. Plugged it in Windows same result nothing happened. Changed the usb cable to usb 3.0 still not mounting. Plugged it and wait till it mounts nothing appeared. So i read and read until I came to this forum and read someone's reply about Paragon NTFS software so I downloaded it from their website and installed then it finally worked.

Hopefully this helps.